1. Field of the Invention
The invention relates to measurement of electromagnetic wavefronts. In particular, it pertains to quantitative, instantaneous measurement of interference light beams produced by a Fizeau interferometer.
2. Description of the Related Art
Light-beam measurements and characterization are important in the manufacture of many optical components, such as data-storage laser heads, for example. Accordingly, many optical interferometric systems have been devised to increase the accuracy and reliability of the measurements. In general, a front-end interferometric device that produces a test and a reference beam is combined with a back-end optical device for resolving phase differences between the beams. This may be achieved simultaneously by encoding (or “marking”) the beams at the front end, such as by using different polarization states for the test and reference beams, and resolving them spatially at the back end. Alternatively, the optical path difference between the test and reference surfaces may be varied in time at the front end, such as by scanning one surface with respect to the other, and the phase differences are resolved temporally at the back end.
One of the problems identified in the prior art has been the ability to properly encode the reference and test beams in the measurement portion of a Fizeau-type interferometer. In U.S. Pat. No. 4,872,755, Kuchel et al. addressed this shortcoming by adopting a different approach to both simultaneous and temporal phase measurement. By introducing an optical delay device in the measurement portion of the interferometer and judiciously selecting the coherence length of the light, the length of the delay path, and the length of the gap in the Fizeau cavity, two coherent test and reference beams as well as two incoherent beams are produced simultaneously. The delay device is used to vary the optical path difference between the coherent beams for temporal phase measurements. Alternatively, the test and reference beams may be polarized after they have been produced in the measurement portion of the device and introduced into a spatially-resolving receiver for simultaneous phase measurements.
Thus, the Kuchel et al. approach requires fine adjustment of the length of the delay path, which is difficult and expensive to implement. In addition, the presence of the two incoherent light beams produces significant background light that may affect the measurements. Therefore, there is still a need for a phase measurement system based on a Fizeau interferometer that does not suffer from these shortcomings.
In U.S. Pat. No. 6,304,330, Millerd et al. describe a back-end system wherein the test and reference wavefronts produced by an interferometer are collimated, divided into sub-wavefronts, phase-shifted, combined to produce interference, and detected along a common axis simultaneously on a single detector or a multiple detector array. The beams can also be detected sequentially on a single detector array, if desired. The Millerd optical system also requires encoded test and reference beams. Therefore, in combination with a front-end Fizeau configuration, the same encoding problems addressed by Kuchel et al. need to overcome. The present invention illustrates a novel approach whereby the output of a Fizeau cavity with a tilted reference mirror is combined with a polarizing element to produce coherent test and reference wavefronts suitable for simultaneous spatial phase measurements in a system as described in U.S. Pat. No. 6,304,330.